This invention relates to a solid state clock for an automobile or the like, and, more particularly, to circuit means between the automobile battery and the display means of the clock.
More specifically, this invention relates to such a circuit means for increasing the voltage from the automobile battery to the clock display means and for maintaining that voltage substantially constant.
Solid state clocks in automobiles or the like have display means in the form of a vacuum fluorescent display device which derives current from a supply such as a 12-volt battery of the automobile. The display normally is energized by a voltage which exceeds that of the automobile battery, normally in the range of 18-20 volts. Thus, multiplying means is provided between the supply and the display, through an integrated circuit or "chip", to increase, normally doubling, the voltage from the battery. The multiplying means comprises an inductor coil or "doubler" which is connected between the supply and the integrated circuit. The integrated circuit, in turn, directs the increased voltage to the vacuum fluorescent display and components thereof. An oscillating switching means in the form of a transistor is provided between the integrated circuit and the multiplying means. However, the multiplied voltage preferably should be maintained substantially constant so as to avoid any fluctuations in the power to the display and integrated circuit.
Heretofore, it has been proposed to employ a second power transistor on the order of four or five watts as a voltage limiter or regulator between the 12-volt supply and the doubler in order to regulate the voltage to the doubler by limiting the same on the order of 111/2 to 121/2 volts. In other words, this power transistor provides direct means between the supply and the doubler to regulate the voltage thereto.
With the present invention, the direct voltage regulator or power transistor having a larger current carrying capacity between the current supply and the doubler is eliminated, and the oscillating transistor between the integrated circuit and the doubler itself is regulated by a second switching transistor and a Zener diode. The second transistor and Zener diode is effective to shut the first transistor off whenever voltage at the doubler increases to a value set by the Zener diode, such as 18 to 20 volts.
In the exemplary embodiment of the invention, a solid state clock for automobiles or the like includes a display means in the form of a vacuum fluorescent display which includes one or more filaments to illuminate the display. Operating power is supplied to the display from an electrical power supply, such as a 12 volt battery for the automobile, through an integrated circuit or chip. The display requires on the order of 18 to 20 volts and, consequently, voltage multiplying means is coupled between the current supply and the integrated circuit. The multiplying means comprises an inductor coil or "doubler" which increases or doubles the voltage from the supply. Converter frequency is supplied from the integrated circuit to the inductor coil through a first transistor which, in effect, runs or operates the doubler.
Means is provided to regulate the duty cycle of the transistor between the integrated circuit and the doubler. This regulating means comprises a second, switching transistor coupled to the base of the aforesaid first transistor, and the base of the second transistor is coupled through a Zener diode to the doubler. The Zener diode has a predetermined value, such as on the order of 18-20 volts, which is equivalent to the voltage needed for operating the display of the clock. As the voltage at the doubler increases to the value set by the Zener diode, the diode turns the second transistor on which is effective to turn the first transistor off, and thereby, regulates the doubler by means of regulating the transistor between the doubler and the integrated circuit.
Other features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawing.